Filter bags made of various materials and of various designs are used in the dust collecting equipment as well as technological devices and are meant for dust separation or product transportation. They are used in almost all branches of industry: energy and fuel, metallurgical, electromechanical, electronic, chemical, light, food, pharmaceutical, wood, paper industries.
The sealing method of a baghouse filter bag's material joints, known from the American patent description no. U.S. Pat. No. 5,156,661 consists in joining the filtration material edges by a seam, and subsequently sealing of the stitched place by covering the seam and both adjacent filtration material edges joined together with a thermoplastic tape, and then welding the tape with the filtration material.
What is commonly used for a filter bag seams' lamination is a polytetrafluoroethylene PTFE tape. A single or multilayered PTFE tape is applied on the outside, working side of the bag, at the stitched place, and then it undergoes high temperature exposure, up to approx. 800° C.
Such a method of a filter bag sealing cannot be applied in case of use of low temperature resistance filtration materials. PTFE laminate can basically be used only for filtration materials made of PTFE fibres or ones covered with PTFE membrane. In case of the remaining filtration materials, made of fibres other than PTFE, seam covering is possible only when using PTFE tape with a glue layer. A disadvantage to such a solution is a low temperature resistance of the glue used in the PTFE tape and also that it cannot be used for joining the majority of the filtration materials made of fibres other than PTFE fibres.
Another method for sealing the places of filtration material joints in a filter bag consists in covering the seams with a silicone layer. The silicone is applied on filter bag elements combined together or on a ready-made filter bag at the stitched places, similarly as in case of the PTFE tape use.
A disadvantage to both solutions where a PTFE laminate or a silicone layer is applied on the outside surface of a filter bag is a low mechanical resistance of the sealing and easiness of its detachment from the bag surface, and as a consequence—a filter integrity loss and likelihood of the product contamination with sealing bits coming off. In the solutions used the sealing layer is applied on a ready-made bag what makes it difficult to cover all the joint points of the filtration material effectively. Besides, in some bag designs, the solutions used do not grant efficient sealing of the inter-material spaces at the places where the edges of the material being joined do not overlap, e.g. in bags with a round bottom sewn in.
Baghouse filter bags can be made of one or several filtration material pieces, joined together permanently with a thread stitch in the way securing the best possible seam tightness.
The simplest design is known from the American patent description no. U.S. Pat. No. 4,959,045. A rectangular filtration material piece is folded along the line parallel to the rectangle's axis which is moved slightly from this axis so that one material edge protrudes slightly beyond the other one after sewing. The protruding edge is bent to cover the shorter edge and finished with an overlock seam.
The bottom part of the bag is combined in the same way, while the top part remains open. In some solutions the open top bag edge is turned inside out and sewn to create a tunnel. A filter bag design frequently used is an open tube shape which is obtained by sewing the longer sides of the rectangular filtration material piece together and sewing in a round element on one side, creating the bottom. A tunnel with inside elements meant for the bag installation in the filtration unit can be sewn to the open top part of the tube. All filtration material joints are sealed with the methods described above.